Alzheimer's Disease (AD), also called senile dementia, is a type of neurodegenerative disease. It has hidden onset and progressive pathology, accompanying typical clinical characteristics of integrated cognitive impairment and personality change. The AD patients will undergo early and recent memory impairment, and subsequently progressive intelligence decline, aphasia, loss of reasoning ability and dyskinesia. This disease will seriously affect the quality of life of the patients and their family members, also incurs a heavy burden on the patient's families and the society.
With the accelerating population aging process, senile diseases have become a significant problems affecting human health. AD, cancer, and cardio- and cerebrovascular accident are three leading causes of mortality in old people. It is believed that AD will be the fourth risk factor to human health in the 21st century. World Health Organization has included AD as one of the five major diseases in 21st century. China entered into aging society in 1999. By the end of 2004, there are 143 million elder people aged 60 and more, comprising 10.97% of the total population. It is predicted that this number will reach 200 million by 2014, 300 million by 2026, and more than 400 million by 2037. The number of elder people will rise to its maximum in 2051 and remains at a scale of 300 million to 400 million. Increasing population of elders leads to increasing incidence of Alzheimer's disease. It is currently reported that more than 20% of people over 80 years are suffered this disease in Europe, Japan and the United States. There are more than 50 million people over 65 years suffered from different types of dementia all over the world.
The main pathological changes of AD are characterized by: extensively and intercellularly formed senile plaque (SP), which results from the deposition of β-amyloid peptide (abbreviated as Aβ or βA); neurofibrillary tangles (NT) resulted from hyperphosphorylated Tau protein in nerve cells; and extensive neuron loss. Lots of evidences indicate that the neurotoxicity of Aβ is the common cause of the diverse etiologies. Therefore, the prevention and/or treatment efforts targeting Aβ have become the focus of AD research in recent years.
Aβ is the metabolite of β-amyloid precursor protein (APP). Under normal conditions, APP generates soluble sAPPα under the enzymatic catalysis of α-secretase. sAPPα have the functions of reducing the concentration of intracellular calcium, regulating synaptic plasticity, promoting synaptic growth and protecting the neuron. This is the main pathway of APP processing, which does not generate Aβ. In the other pathway, APP generates sAPPβ and C99 under the enzymatic catalysis of β-secretase. C99 then releases Aβ peptide under the enzymatic catalysis of γ-secretase. The hydrolysis of C99 by γ-secretase is a heterogeneous process: an enzymatic cleavage between alanine 713 and threonine 714 generates Aβ1-42; and an enzymatic cleavage between valine 711 and isoleucine 712 generates Aβ1-40 (Selkoe D J. Alzheimer's disease: genes, proteins, and therapy. Physiol Pev, 2001, 81 (2): 741-766). Aβ1-42 accounts for about 10% of total Aβ protein, Aβ1-40 accounts for about 90%. However, Aβ1-42 is more likely to aggregate, and the aggregated Aβ1-42 constitutes the basic component of senile plaques.
The primary structure of Aβ1-42 peptide is shown as follows (SEQ ID NO:4):
Asp-Ala-Glu-Phe-Arg-His-Asp-Ser-Gly-Tyr-Glu-Val-His-His-Gln- 1               5                  10                  15 Lys-Leu-Val-Phe-Phe-Ala-Glu-Asp-Val-Gly-Ser-Asn-Lys-Gly-Ala- 16             20                  25                  30  Ile-Ile-Gly-Leu-Met-Val-Gly-Gly-Val-Val-Ile-Ala 31               35                  40     42
Ten amino acid residues on C-terminal of Aβ1-42 (residues 33 to 42) and residues 17 to 21 are highly hydrophobic and constitute the hydrophobic region of Aβ1-42; amino acid residues 28-42 have more probability to form a β-sheet conformation, while the amino acid residues 9-21 may also form a β-sheet conformation. B-sheet conformation is conducive to aggregation of Aβ1-42 peptide. Experimental results show that three residues on C terminal, Val40Ile41Ala42, stabilize the β-sheet conformation and are conducive to formation of a β-sheet. N-terminal of Aβ 1-42 peptide is hydrophilic, and can form conformation of α-helix, random helix or β-sheet, depending on the different solution conditions. It has been shown that the β-sheet conformation is conducive to the aggregation of Aβ peptide, and the aggregation of Aβ peptide is due to the interaction between its hydrophobic regions. Soto et al. (Soto C, Kindy M S, Baumann M, et al., Inhibition of Alzheimer's amyloidosis by peptides that prevent beta-sheet conformation. Biochem. Biophys. Res. Commun. 1996, 226(3):672-680) replaced amino acid residues adjacent to the hydrophobic regions of Aβ peptides by proline, the resulting small peptides will not form a β-sheet conformation, and binds to Aβ peptide instead, allowing to maintain Aβ peptide in its random helical conformation and inhibit its aggregation. It has also been shown that one hydrophobic region of Aβ peptide, a pentapeptide Aβ16-20 (Lys-Leu-Val-Phe-Phe (SEQ ID NO:6)), can binds to Aβ peptide and thus prevent aggregation. It is demonstrated by replacement with alanine one by one that Lys16, Leu17 and Phe20 play a critical role in this process. This shows that the Aβ16-20 residues constitute the section by which two adjacent Aβ peptides bind to each other in the aggregation of Aβ peptides. It has been shown that the conformation of Aβ significantly affects its aggregation capabilities: when the major secondary structure is α-helix, the aggregation is slower; whereas the major secondary structure is β-sheet, the aggregation is more quickly.
Under certain conditions, the exposure of β-sheet-rich hydrophobic regions will lead to the aggregation of Aβ and the formation of oligomers, and eventually the formation of insoluble material deposited in the intercellular space of neurons. This may results in neurotoxicity and the increased activities of glial cells in brain, producing inflammatory mediators and complement which may together form amyloid plaques.
The increased hydrophobic charge of a molecule is one of the major causes of the Aβ aggregation. As compared with Aβ1-40, Aβ1-42 has two more residues, which not only increase the hydrophobicity of Aβ, making it easier to aggregate, but also improve the stability of aggregates, allowing them to selectively deposit in amyloid plaques in early stage. Aβ1-42 may be the initial factor of the process which forms oligomers, fiber and the plaque from soluble Aβ. (Younkin, S. G. 1995. Evidence that A beta 42 is the real culprit in Alzheimer's disease. Ann. Neurol. 37: 287-288; Matsuok Y, Saito M, Lafrancois T, et al. Noval therapeutic approach for the treatment of Alzheimer's disease by peripheral administration of agents with an affinity to β-amyloid[J]. J Neurosci, 2003, 23(1): 1-5).
Jarrett et al. speculated that Aβ1-42 served as a “seed” to initiate the deposition of Aβ. Other monomer then gradually gathered around the nucleus and extended the peptide chain to form fibrils (elongation), and the fibrils further spread and finally formed plaques. (Jarrett J T, Lansbury P T Jr. Seeding “one-dimensional crystallization” of amyloid: a pathogenic mechanism in Alzheimer's disease and scrapie? Cell. 1993, (6): 1055-1058).
It is believed that the aggregation and deposition of Aβ and the formation of senile plaque and the accompanying neuron damage constitute the central parts of the pathological mechanisms of AD. Therefore, inhibiting the aggregation of Aβ peptide and promoting the degradation and clearance of Aβ peptide may be the fundamental means of preventing and/or treating AD.
Recent studies on development of drugs for Aβ focus on the aspect of reducing the formation of Aβ, increasing the Aβ clearance, preventing or reversing Aβ aggregation, and inhibiting the toxicity of Aβ, and the like. Researchers in Medicine School of Washington University found that AD mice could surprisingly restore their brain cell functions after clearance of amyloid plaques in their brain, which indicates a promising prospect of drugs targeting Aβ. Among a variety of drugs against Aβ, β-sheet blockers draw increasing attention of the researchers.
There are two main types of drugs belonging to β-sheet blockers:
(1) Based on the realization that the low molecular weight amino-glycoprotein (gly-cosaminoglycan, GAGs) could stabilize the β-amyloid plaques and inhibit the degradation of the plaque, Neurochem Inc. in Canada designed and synthesized GAG derivatives. In vivo experiments showed that such low molecular weight GAG analogues can significantly reduce levels of β-amyloid in plasma and brain and inhibit Aβ aggregation, and thus can be used for the treatment of AD. One such low molecular weight GAG analogue, designated as Alzhemed, has been in clinical trials phase III.
(2) Chacón et al. induced behavior disorder in rats by intraperitoneal injection of fibrotic Aβ, and then assessed β-sheet breaker peptide consisting of five amino acid residues (five-amino-acid beta-sheet breaker peptide, iAbeta5p) for their protection effects on neuron. Results showed that iAbeta5p could not only inhibit the formation of Aβ fibrils, but also degrade Aβ fibrils (Chacón M A, Barrïa M I, Soto C, et al., Beta-sheet breaker peptide prevents beta-induced spatial memory impairments with partial reduction of amyloid deposits. Mol. Psychiatry. 2004, 9(10): 953-61). The drug of iAbeta5p has been in clinical trials phase III.
In the art, there remains need of new active drugs which can specifically bind to monomer of β-amyloid (Aβ1-42), stabilize its normal spatial structure, inhibit its formation of β-sheet, and prevent the formation of soluble β-amyloid oligomers and β-amyloid plaque. Said drug should be able to inhibit the aggregation of Aβ peptide, promote the degradation and clearance of Aβ peptide, and thus can be used for prevention and/or treatment of AD.